The present invention relates to a decanter centrifuge for separating slurry into liquid and solid cake, and more specifically to a centrifuge for washing the cake effectively.
In general, a decanter centrifuge having the capability of draining and washing is widely known, and called a screen bowl type decanter centrifuge.
Referring now to FIG. 7, the typical screen bowl type decanter centrifuge will be described.
The decanter centrifuge comprises a bowl 100 and a screw conveyor 200 rotating in the bowl and rotating relatively thereto.
The bowl 100 comprises a large diameter section 101, a tapered section 102, and a small diameter section 103 formed in a single piece. A clear liquid discharging port, or a dam 104 is formed on the end surface of the large diameter section 101, and cake discharging ports 105 are formed in the vicinity of end portion of the small diameter section 103. The small diameter section 103 is provided with filtrate discharging holes 107 formed on its circumferential wall 108. The inner peripheral surface of the circumferential wall 108 is covered with a porous material 106.
The screw conveyor 200 rotating at a constant differential speed with respect to the bowl 100 comprises a hub 201 as an axis of rotation and a flight 202 fixed on the hub 201. A washing fluid chamber 203 is provided within said hub 201, and washing fluid spray nozzles 204 are provided at the position corresponding to the washing fluid chamber 203.
Slurry, which is an object to be processed, is supplied to the large diameter section 101 through an unrotatable slurry supply pipe 300 within the hub 201. Washing fluid passes through a washing fluid supply pipe 301 provided around the slurry supply pipe 300 and is supplied to the washing fluid chamber 203 described above through a washing fluid supply port 302.
Slurry supplied through the slurry supply pipe 300 to the large diameter section 101 is pressed against the inner peripheral surface of the large diameter section 101 by centrifugal force. Liquid component in the slurry is discharged from the dam 104 formed at the end portion of the large diameter section 101, and the cake in the slurry is transported by the flight 202 through the tapered section 102 and the small diameter section 103, and discharged from the cake discharging ports 105. The cake in the small diameter section 103 is drained while being washed by the washing fluid.
However, in the related art as described thus far, since the cake is forced to be transported by the flight 202 while being formed into generally triangle in cross section with one side situated on the surface of the flight 202 facing toward the cake discharging ports 105, even when the washing fluid is sprayed onto the portion of generally triangle in cross section, most part of the washing fluid just flows over the tilted surface of the cake and does not penetrate into the cake, and thus the effect of the cake washing cannot be expected much. When a large quantity of the washing fluid is supplied to enhance the washing effect, the amount of waste liquid increases as well, thereby hindering the draining effect. Therefore, the filtering section has to be extended to produce a satisfactory draining effect, and as a consequence, the whole length of the apparatus increases, and thus the cost of the apparatus increases as well.
In the related art, an attempt has been made to provide a plate or a knife between the flights at a prescribed distance from the bowl and break the accumulation of the cake before spraying the washing fluid in order to enhance the effect of the cake washing. However, since these parts may resist transportation of the cake, a larger power is required. In addition, since washing of the machine itself is difficult, the accumulated cake may cause so called a blockage and thus the object to be processed cannot be processed in volume stably. In addition, in the related art described above, the number of components increases and thus the cost of the apparatus also increases.
With the problems described above in view, it is an object of the present invention to provide a decanter centrifuge that can enhance the effect of the cake washing without increasing the quantity of the washing fluid supplied and without increasing the number of components.
In order to achieve the object described above, the decanter centrifuge of this invention comprises:
a bowl;
a screw conveyer provided in the bowl so as to rotate relatively to the bowl;
the bowl comprising a cylindrical large diameter section into which slurry is supplied, a tapered section having the diameter decreasing from the large diameter section, and a small diameter section connected to the side of the tapered section having a smaller diameter;
the small diameter section being provided with fine filtrate discharging holes formed on a part or all over the circumferential wall thereof and with cake discharging ports at the end opposite from the large diameter section;
the small diameter section being provided with washing fluid supply means for supplying washing fluid to the cake transported from the large diameter section through the tapered section;
wherein the screw conveyor includes a plurality of flights for transporting the cake to the cake discharging ports in the small diameter section;
the plurality of flights includes a small clearance portion defining a small clearance and a large clearance portion defining a large clearance between the tip of the flight and the inner peripheral surface of the small diameter section respectively;
the whole region of the large clearance portion of the plurality of the flights overlaps with the small clearance portion of another flight located at the same axial position of the screw conveyer.
The large clearance means a clearance of at least double the small clearance in size.
In the decanter centrifuge, the washing fluid supply means is preferably provided in the large clearance portion on the side facing to the large diameter section. In addition, the washing fluid supply means is preferably provided so as to be able to spray the washing fluid onto the surface of the large clearance portion of the flight on the side facing to the large diameter section.
In the decanter centrifuge, the large clearance portion of the flight and the small clearance portion of another flight located at the same axial positions are preferably provided in such a manner that when the large clearance portion makes at least a half turn, the small clearance portion passes through the same point.
In addition, in the decanter centrifuge described above, the depth of the large clearance and the length of the large clearance portion are preferably defined in such a manner that cake located at the large clearance portion on the side facing to the cake discharging ports passes through the large clearance formed between the inner peripheral surface of the small diameter section and the large clearance portion while forming a cake residue layer having the same thickness as the large clearance on the side of the large clearance portion facing to the large diameter section, and the contact pressure from cake is not applied to the surface of the large clearance portion of the flight facing to the cake discharging ports immediately before the large clearance portion ends and the small clearance portion starts.
As is described thus far, according to the present invention, since a large clearance portion is formed on the flight, and the washing fluid is supplied onto the thin cake residue layer having passed therethrough, the washing effect can be enhanced without increasing the number of components or the amount of the washing fluid. Especially, the decanter centrifuge in which the washing fluid is supplied to the surface of the large clearance portion of the flight on the side facing to the large diameter section can further enhance the washing effect since the washing fluid is supplied uniformly onto the whole surface of the thin cake residue layer.